1. Field of the Invention
This invention relates to a novel photosensitive polyamide resin composition. More particularly, it relates to a polyamide resin composition for use as a relief printing material having high image-reproducing properties and exhibiting good softness and elasticity.
2. Description of the Prior Art
As the process for preparing a photosensitive polyamide resin composition having high image-reproducing properties, there has been proposed a process in which a polyamide is reacted with a compound having one vinyl group and one epoxy group to obtain a photosensitive resin consisting of a polyamide having double bonds introduced therein by this reaction (see Japanese Patent Publications No. 16,062/72, No. 45,583/72 and No. 23,304/74).
The sensitivity of a photosensitive resin comprised solely of a polyamide having double bonds is low because the mobility of the polyamide is low, and therefore, to obtain high image-reproducing properties, the exposure time to actinic rays must be prolonged considerably. Furthermore, this resin is defective in that the wash-out speed is very slow in the non-illuminated non-image area. As means for eliminating these defects, there have been proposed processes in which a monofunctional vinyl monomer is incorporated as a photo-insolubilizer into a polyamide having double bonds introduced therein (see Japanese Patent Publication No. 45,585/72 and No. 13,442/74 and Japanese Laid-Open Patent Application No. 40,349/74). For example, Japanese Patent Publication No. 45,585/72 teaches that a monofunctional vinyl monomer having a terminal hydroxyl group, such as hydroxyalkyl acrylate or methacrylate, is used as the photo-insolubilizer. However, since the crosslinked structure formed by photo-polymerization in this system is made of the monofunctional vinyl monomer and the polyamide having double bonds introduced therein, the crosslinked structure is not three-dimensional and the structure has poor resistance to developer solvent. Accordingly, the problem of dissolving-out and the collapse of dots during development arises, with the collapse of the relief image being especially conspicuous when the development time is prolonged. As means for eliminating these defects, there is known a composition in which a polyfunctional vinyl monomer is incorporated as the photo-insolubilizer as proposed in Japanese Patent Publication No. 13,442/74 and Japanese Laid-Open Patent Application No. 40,349/74. However, since ordinary polyfunctional vinyl monomers of the polyether, polyester or bis-acrylamide type disclosed in Japanese Laid-Open Patent Application No. 40,349/74 have poor compatibility with polyamides, the polyfunctional vinyl monomers tend to separate and appear on the surface of the photosensitive layer during storage tests. Accordingly, this composition cannot be used as a photosensitive resin relief material where a long-term guarantee of quality is required.
Japanese Patent Publication No. 13,442/74 proposed use of a polyfunctional vinyl monomer obtained by reacting at least one member selected from unsaturated carboxylic acids, unsaturated alcohols, primary amines and secondary amines with glycidyl arcylate or methacrylate, which is represented by the following general formula: ##STR2## wherein p and m are numbers satisfying requirements of p+m=k and 1.ltoreq.m.ltoreq.k, n is a number of from 1 to 10, R stands for H or an alkyl group, and A stands for a residue derived from an unsaturated carboxylic acid, an unsaturated alcohol, a primary amine or a secondary amine.
The above-mentioned polyfunctional vinyl monomer is superior to the ordinary polyfunctional vinyl monomers proposed in Japanese Laid-Open Patent Application No. 40,349/74 in compatibility with polyamides. It is believed that this is due to the --OH group formed by the reaction of the active hydrogen atom of the unsaturated carboxylic acid, unsaturated alcohol, primary amine or secondary amine with the epoxy group of glycidyl acrylate or methacrylate. However, since the unsaturated carboxylic acid, unsaturated alcohol or secondary amine has only one active hydrogen atom, only one molecule of glycidyl acrylate or methacrylate is added to such active hydrogen-containing compound and the formed polyfunctional vinyl monomer contains only one --OH group in the molecule. Accordingly, this polyfunctional vinyl monomer is still not sufficiently compatible with polyamides. Where the resulting composition is subjected to an accelerated storage test at a high relative humidity exceeding 80% or a high temperature of 35.degree. to 40.degree. C., the polyfunctional vinyl monomer still tends to separate from the polyamides. Furthermore, if a relief material formed of this composition is stored in a room for a long time in summer, the polyfunctional vinyl monomer appears on the surface of a relief material. When the primary amine is reacted with glycidyl acrylate or methacrylate two hydroxyl groups are formed, since the primary amine has two active hydrogen atoms in the molecule. This improved the compatibility with polyamides and the polyfunctional vinyl monomer does not separate even during an accelerated storage test conducted under high humidity and high temperature conditions. When a primary amine is used, double bonds are combined through linkages as shown by the following formula: ##STR3## wherein R and R' each stand for H or CH.sub.3, and X stands for a residue of the primary amine.
As is apparent from the above formula, two molecules of glycidyl acrylate or methacrylate are combined with one nitrogen atom. The molecular weight between the two double bonds is lowest, a value of 272, when two molecules of glycidyl acrylate are added, and is highest, a value of 300, when two molecules of glycidyl methacrylate are added. Since the molecular weight between the two double bonds is relatively low, i.e. in the range of from 272 to 300, and only chemical linkages which are relatively rigid, such as ester linkages or epoxy ester linkages, are present between the two double bonds, the crosslinked structure formed by photo-polymerization of the above-mentioned polyfunctional vinyl monomer is very rigid. Furthermore, the polyamide chain is completely included in the crosslinked structure through chemical linkages because double bonds are introduced in the polyamide. Therefore, an exposed relief plate using such composition is very hard and brittle. With a hard and brittle relief plate, serious problems, such as separation of the plate from the relief drum and cracking of the relief, occur during the printing operation.